1. Field of the Invention
The present invention generally relates to data synchronization. More particularly, the invention relates to data synchronization across a network.
2. Description of Related Art
For reasons of ubiquity, capacity, efficiency and cost, Ethernet is regarded as the unifying transport layer of choice for network convergence. Ethernet backhaul will become universal over time and may replace traditional TDM backhaul.
For most operators Ethernet will be introduced on the back of existing TDM network connections given the huge investment in TDM network infrastructure. This will typically involve gradual migration using data overlay, with a decision at some future point to change to an all packet-based network.
A key consideration when engineering a change to Ethernet is synchronization. In particular, operators must determine how to replace the synchronization that typically occurs with Plesiochronous Digital Hierarchy (PDH) and Synchronous Digital Hierarchy (SDH) network connections. Operators may also consider how to provide synchronization of phase which is not supported by PDH and SDH network connections but is required for 3G and 4G/LTE TDD base stations.
Prior art methods to achieve clock sync transport over an all-IP network requires expensive new technology (e.g., in the form of packet delivery conforming to the IEEE1588 PTP standard). The prior art suffers from packet delay variation and/or asymmetric delays that present a problem for the PTP protocol especially with high user traffic loading conditions. Additionally, carrying PTP clock information along with user payload consumes valuable bandwidth.
For example, synchronous Ethernet and IEEE 1588v2 have been used in the prior art. Synchronous Ethernet uses the physical Ethernet layer to transport clock synchronization. Its operation is similar to that used in TDM networks, where a clock signal is injected into the bit stream to lock the physical layer clock. Each device in the network recovers, cleans, and then distributes the clock to its downstream neighbor. For end-end clock delivery, every intervening node (switch/router) within the network must support Synchronous Ethernet which requires expensive upgrades. Further, IEEE standardized Synchronous Ethernet cannot distribute phase alignment or time of day.
Packet-based IEEE 1588v2 performs frequency and phase synchronization, but, unfortunately, has traffic loading issues. IEEE 1588v2 is a Precision Timing Protocol (PTP) which provides a packet-based timing mechanism for phase and frequency synchronization. Dedicated timing packets are transmitted within the data packet stream to maintain a Master-Slave synchronization relationship. Time-stamped PTP packets are sent from the master clock to the slave clocks and from the slave clocks back to the master. A timing recovery algorithm uses these packets to calculate and offset the delays and differences in delay (packet delay variation or PDV) across a network.